1. Field of the Invention
The present invention pertains generally to the field of technical ceramic materials and more specifically to a high performance monolithic silicon nitride material used as a ceramic insulator in corona discharge igniter systems for internal combustion engines and gas turbines.
2. Prior Art
Due to the recently rising prices of oil, as well as legislation for emission control, increases in fuel efficiency in internal combustion engines have become extremely important. Governments in many countries have mandated automobile manufacturers to reach increased gas mileage for vehicle models manufactured in the near future.
One way to increase the fuel efficiency is to change the current state of the art of spark plugs which ignite the fuel in the internal combustion engine by generating a spark (plasma) between narrow gap electrodes separated by an air-fuel mixture, by more energy efficient corona discharge igniter (CDI) systems. In a CDI system, instead of a short duration spark, a steady corona discharge is generated between electrodes and around the ceramic insulator tip using a radio frequency electromagnetic field. The generation of a corona discharge does not require a dielectric breakdown of the air-gas mixture to occur to ignite the fuel. The corona is therefore generated at lower voltages than required for a spark. The corona generates a steady fuel burning front that is easier to control and increases the fuel efficiency. It also allows ignition of lean fuel mixtures that burn cleaner but are difficult to ignite using spark-plugs.
One CDI system is described in U.S. Pat. No. 6,883,507 (Freen), and it specifies that a feed-thru insulator is used for the system to work. The patent advises the use of BN (boron nitride) for this purpose (line 35-36, column 6), and does not provide additional guidance for the nature of insultor material.
U.S. Publication No. 2011/0175691 A1 (Smith et al) describes a compact electromagnetic device generating a corona discharge in a coaxial resonating cavity that can be used to ignite combustible materials in combustion engines. In their invention, an insulated guide is required which is shown in FIG. 5 (item 510) and FIG. 6 (item 610), however the material used is not specified in any way in the disclosure.
U.S. Publication No. 2011/0247579 A1 (Hampton et al) descibes a corona igniter with an enhancing electrode tip composed of metal shapes at the tip of the insulator. The type of insulator is only described as “an insulator that surronds the electrode body portion and extends from the insulator tip to to insulator upper end” (paragraph [0086]).
U.S. Publication No. 2010/0282197 A1 (Permuy et al) describes in detail the preferred shapes of the feed-thru insulator necessary for this CDI system and specifies that the insulator should be a ceramic material. Same application notes (first three lines of paragraph [0006]) that, although BN is suggested in U.S. Pat. No. 6,883,507 for the insulator due to its high dielectric breakdown strength and a low dielectric constant, that the material is very soft, expensive and difficult to form into required insulator shapes. It is also noted that relative permittivity (i.e. dielectric constant) should be low for the material to have a high dielectric strength (paragraph [0007]), but offers no specific insulator materials that have these properties.
U.S. Publication No. 2010/0175655 A1 (Lykowski et al) describes further that the ceramic insulator (which can be combined with a non-ceramic insulator) is an aluminum or silicon (paragpaph [0037] lines 2 and 4) containing oxide and or nitride based ceramic with up to 5% additions of calcium oxide, magnesium oxide, zirconium oxide, boron oxide or boron nitride additions to alumina or silica (paragraph [0039]). Lykowsky et al also disclose that the desired dielectric strength of the ceramic insulator should be above 15 kV/mm (or more preferably 17 kV/mm or above and most preferably above 19 kV/mm) (paragraph [0051]). Additionally, the application describes that the ceramic material should have a modulus of rupture strength (MOR) of at least 100, 200 or 400 MPa (in increasing order of preference), low dielectric constant at 1 MHz (lower than 9—paragraph [0069]), and low loss tangent (most preferably less than 0.005 at 1 MHz—paragraph [0053]). This patent application does not provide any examples of inventive compositions their invention requires. Specific compositions involving silicon nitride, how they can be produced or what they consist of when produced are also not disclosed. The inventors state that alumina, silicon nitride and aluminum nitride of their invention meet the listed properties, but do not provide any data or examples to support any of the properties required by the invention, for any of the composition ranges for any of the materials described.
U.S. Pat. No. 8,053,966 (Walker Jr.) discloses a method of manufacturing Al2O3 (alumina) ceramic spark plug insulators. In the background portion of the disclosure the inventor states (paragraph [0010]) that typical dielectric strength (RMS) of alumina spark plug materials is about 400 V/mil or 1560 V/mm, substantially lower than what is recommended for CDI in U.S. Publication No. 2010/0175655 A1 above. In the Summary of U.S. Pat. No. 8,053,966, the inventor states (Paragraph line 3) that “high purity” alumina (purity is not defined) dielectric strength can be 475 V/mm or 1852 V/mil, but that this material is difficult to process and is not adequate for conventional spark-plug insulator manufacturing. None of the other properties listed as important are disclosed for any of the claimed materials by the inventor. The inventor specifically references U.S. Pat. Nos. 4,879,260 (Manning) and 7,169,723 (Walker Jr.) in which the additions of Zr, Ca, Si, Mg, Ca, B oxides and BN are added to alumina ceramics in order to improve the dielectric strength of alumina. The inventors of U.S. Pat. No. 8,053,966 and U.S. Pubication No. 2010/0175655 A1 also do not recognize that the listed combination of additives (MgO, CaO, ZrO2 and B2O3 and BN) (sintering aids) described in U.S. Publication No. 2010/0175655 A1 are not effective in obtaining dense sintered silicon nitride or obtaining desired properties for silicon nitride, although they may be so for alumina materials.
Sintering aids, their levels and methods of processing required for effective sintering of silicon nitride and the combination of the sintering aids that result in desirable material properties for corona discharge ignition systems are not given in the above mentioned prior art, and the ones discussed therein would not result in adequate materials.
U.S. Pat. No. 5,358,912 (Freitag et al) discloses Barium Aluminum Silicate in situ reinforced silicon nitride that is pressureless sintered and contains about 3% porosity (Table 1) after sintering, which is too high to achieve high dielectric strengths for CDI insulators. Although some of the compositions show low delectric constants at 35 GHz, values at 1 MHz (important for the CDI application) are not disclosed. It is known that dielectric properties are especially frequency dependent. Dielectric loss tangents are not disclosed. Therefore, improvements are required for materials of this invention to be considered for a CDI application.
U.S. Publication No. 2006/0014624 A1 (Mikijelj) discloses silicon nitride compositions which result in very high dielectric strengths and low electrical conductivity, but do not identify other properties that need to be satisfied in order for the said silicon nitride to be used in a corona discharge ignition system or how efective it may be in that application.
From the description of the prior art above it can be seen that materials of the prior art (alumina, silica, boron nitride and aluminum nitride) for the use in CDI systems are not adequate and have deficiencies in some of their properties. Dielectric strength is low (alumina), dielectric constant and loss tangent is high (alumina and aluminum nitride e′ is 9 or above), the material is too soft (boron nitride), the material mechanical strength and fracture toughness are low. Although silicon nitride is mentioned in the prior art in general terms, none of the prior art recognizes the sintering aids or their combination and amounts and ratios necessary to obtain the combination of properties required for the CDI system to work efficiently. The sintering aid system suggested in the prior art, in fact, does not work for silicon nitride. What is therefore still needed is an invention that provides the composition ranges and defines the ceramic material required for the properties to be met as well as how these materials can be made.